This application relates to polarization-mode dispersion in optical fibers, and more specifically, to techniques and systems for emulating polarization-mode dispersion in optical fibers.
Some optical transmission media such as optical fibers may be birefringent to exhibit different refractive indices for light of different polarizations. Typical causes for such birefringence in certain fibers include, among others, imperfect circular core and unbalanced stress in a fiber along different transverse directions. The axis of birefringence of the optical fiber change randomly on a time scale that varies between milliseconds and hours, depending on the external conditions. Optical fibers with such birefringence, which evolves randomly along the fiber, are said to exhibit polarization-mode dispersion (xe2x80x9cPMDxe2x80x9d). Therefore, an optical signal, that comprises of two components along the two orthogonal principal polarization states for each frequency, can be significantly distorted after propagation through the transmission medium. The amount of PMD may be characterized by the average differential group delay (xe2x80x9cDGDxe2x80x9d) between two principal states of polarization.
This polarization-mode dispersion is undesirable because the pulse broadening can limit the transmission bit rate, the transmission bandwidth, and other performance factors of an optical communication system. In fact, PMD is one of key limitations to the performance of some high-speed optical fiber communication systems at or above 10 Gbits/s due to the fiber birefringence. Fibers with significant PMD (e.g., about 1 to 10 ps/km1/2) are used in various fiber networks, particularly in those that were deployed in 1980""s and early 1990""s. Hence, it is desirable to characterize various effects of PMD in fiber systems, including high-speed transmission that uses those PMD fibers.
One difficulty to characterize effects of PMD in fiber systems is that high PMD fibers that are already used in many fiber systems are no longer commercially available. Hence, PMD emulating devices have been developed and used to emulate actual PMD in a fiber system. Such PMD emulator may be used during testing and design phases of high-performance fiber systems, allowing for rapid and convenient exploration of a large number of different realizations of instantaneous DGDs of a fiber with PMD. For example, a PMD compensator may be designed by using the PMD emulating device to test its PMD compensating capability.
The DGD in an actual PMD fiber, however, is not a fixed value but is a random variable that has a Maxwellian probability density function. See, e.g., Gisin et al., xe2x80x9cExperimental Investigations of the Statistical Properties of Polarization Mode Dispersion in Single Mode Fibers,xe2x80x9d IEEE Photonics Tech. Letters, Vol. 5, No.7, pp.819-821, July 1993. One difficulty in designing a PMD emulator is to produce a probability density function for the DGD values that substantially resembles a Maxwellian probability density function for any wavelength within a desired spectral range.
One embodiment of a PMD emulator of this disclosure includes a plurality of birefringent wave-guiding sections to transmit light and a plurality of variable polarization-changing connectors to connect the wave-guiding sections. The wave-guiding sections are configured to respectively produce different relative delays between two orthogonal principal polarizations. Polarization-mode dispersion in an actual fiber can be emulated by transmitting a beam through the wave-guiding sections and the connectors.
Each connector is coupled between two adjacent sections to transmit light and is operable to variably modify a polarization of light. Different connectors can be adjusted to produce different modifications in the polarization of light. The number of the sections, the different relative delays in the sections, and different polarization modifications between different adjacent sections are selected to produce a substantially Maxwellian probability density function for different total delay values between two orthogonal principal polarizations.
The connectors can be adjusted to produce different sets of polarization modifications so that the Maxwellian probability density function can be emulated for any fixed frequency at different wavelengths with a desired spectral range, e.g., the ITU wavelengths for WDM systems.